The invention relates to a method of operating a drive train of a motor vehicle with a motor whose torque is transmitted to the drive wheels via the drive train which includes an unsynchronized main transmission and a synchronized output drive unit.
EP 0 676 566 A1 discloses a method for operating a motor vehicle having an engine and a transmission with an unsynchronized main transmission and a synchronized output drive unit. Arranged between the drive motor and the transmission is a clutch which is actuated by external force. The necessary adaptation of the rotational speed of a transmission input shaft, the so-called synchronization to a synchronous rotational speed of the target gear, in the event of a shift from an original gear to a target gear in the main transmission is carried out by means of the drive motor. Here, the synchronous rotational speed of the target gear is that rotational speed of the transmission input shaft which is generated at the momentary speed of the motor vehicle with the target gear selected.
The synchronization cannot be carried out reliably if the drive motor is not fully functional, that is to say is in an emergency mode. Since the main transmission is embodied as an unsynchronized transmission, there is the risk that the original gear is disengaged but the target gear cannot be engaged. There is therefore no drive connection between the drive motor and driven vehicle wheels. The drive motor therefore cannot be utilized either to accelerate or decelerate the motor vehicle.
It is the object of the present invention to provide a method of operating a drive train including an unsynchronized main transmission and a synchronized output drive unit in such a way that smooth engagement of a target gear can be ensured.